Aluminum-copper-lithium alloys are under consideration as replacements for conventional aluminum alloys in launch systems. Currently, launch vehicles are constructed primarily from Aluminum Association registered alloys 2014 (Titan) and 2219 (Space Shuttle External Tank). Most of the dry weight of such launch systems, i.e., excluding propellant, is in propellant containment. For state of the art systems such as the Space Shuttle External Tank and the planned Titan IV cryogenic upper stage, the preferred propellant system is liquid hydrogen and liquid oxygen, which are each cryogenic liquids. It is therefore important for the structural alloy for such propellant containment to have both high strength and high toughness at cryogenic service temperatures. Furthermore, it is particularly advantageous for the alloy to have substantially equal or greater strength and toughness at cryogenic temperatures than at ambient temperature in both the parent alloy and any weldments. The ability to achieve higher fracture toughness and strength at cryogenic temperatures enables the structural proof test for the tank to be conducted more inexpensively at ambient rather than at cryogenic temperatures. If both strength and toughness are substantially the same or greater at cryogenic temperatures, a successful room temperature proof test ensures that neither strength-overload-induced nor toughness-limited-induced failure will occur at cryogenic service temperatures.
Cold work induced after solution heat treatment and quenching but before artificial aging is known to affect the mechanical properties of Al--Cu and Al--Cu--Li alloys. The most common way to induce such cold work is by plastically stretching axisymmetric product forms such as extrusions, sheet, and plate. The stretch, typically performed at room temperature, serves the dual function of straightening the product by plastic offset and providing dislocations that serve as nucleation sites for high-aspect-ratio strengthening precipitates, e.g., platelets, laths, etc., thereby increasing strength. Stretch is also known to increase room temperature toughness in Al--Cu and Al--Cu--Li alloys, but its effect on cryogenic toughness has not been reported to our knowledge.
Several aluminum-copper-lithium alloys have been commercialized. These include Aluminum Association (AA) registered alloys 2020, 2090, 2091, 2094, 2095, 2195, and 8090.
Alloy 2020 has a nominal composition, in weight percent, of Al--4.5Cu--1.1Li--0.5Mn--0.2Cd and was registered in the 1950's. Although the alloy possessed a relatively low density and developed high strength, it also possessed very low levels of fracture toughness and ductility. These problems along with processing difficulties led to the withdrawal of the alloy from the Aluminum Association register.
Alloy 2090 comprising Al--(2.4-3.0)Cu --1.9-2.6)Li--(0-0.25)Mg--0.12Zr was designed as a low density replacement for high strength alloys such as 2024 and 7075. Although this alloy develops relatively high strength, it also possesses poor short transverse fracture toughness and poor short transverse ductility associated with delamination problems and has not yet had wide range commercial success.
Alloy 2091 comprising Al--(1.8-2.5)Cu--(1.7-2.3)Li--(1.1-1.9)Mg--0.12Zr was designed as a high strength, high ductility alloy. However, at heat treated conditions that produce maximum strength, ductility is relatively low in the short transverse direction. Additionally, the strength achieved by alloy 2091 in non-cold-worked tempers is below the strength attained by the alloy in cold-worked tempers.
Alloy 8090 comprising Al--(1.0-1.6)Cu--(2.2-2.7)Li--(0.6-1.3)Mg--0.12Zr was designed for aircraft applications in which exfoliation corrosion resistance and damage tolerance were required. However, alloy 8090's limited strength capability and poor fracture toughness have prevented the alloy from becoming a widely accepted alloy for aerospace and aircraft applications.
Alloy 2094 comprises Al--(4.4-5.2)Cu--(0.8-1.5)Li--(0.25-0.6)Mg--(0.25-0.6)Ag--0.25max. Zn--0.1max.Mn--(0.04-0.18)Zr, while alloy 2095 comprises Al--(3.9-4.6)Cu--(1.0-1.6)Li--(0.25-0.6)Mg--(0.25-0.6)Ag--0.25max.Zn--0.10 max.Mn--(0.04-0.18)Zr. Alloy 2195 is similar to alloy 2095, but has slightly lower Cu and Li limits. These alloys possess exceptional properties such as ultra-high strength, high modulus, good weldability, etc.
U.S. Pat. Nos. 5,032,359 and 5,122,339 and U.S. patent application Ser. Nos. 07/327,666 filed Mar. 23, 1989, 07/493,255 filed Mar. 14, 1990 and 07/471,299 filed Jan. 26, 1990, each of which are hereby incorporated by reference, disclose aluminum alloys containing copper, lithium, magnesium and other alloying additions. These alloys have been found to possess very favorable properties such as high strength, high modulus, good weldability and good natural aging response.
In view of the technological importance of using improved alloys at cryogenic temperatures, it would be desirable to provide a low density, aluminum-base alloy that has higher strength and fracture toughness relative to conventional aluminum alloys and both increased strength and increased fracture toughness at cryogenic temperatures in comparison to room temperature. The present invention has been developed in view of the foregoing and provides aluminum-copper-lithium alloys within defined compositional ranges that exhibit improved combinations of cryogenic fracture toughness and strength when processed in accordance with the method of the present invention.